T The petroleum industry faces the challenge of declining oil recovery, as primary and secondary methods generally produce only about one-third of the initial oil in place, leaving a large portion of the oil trapped within the rock pores due to capillary forces and the high interfacial tension between oil and water. one approach to address this problem is chemical enhanced oil recovery (eor), particularly surfactant flooding. surfactants work through two main mechanisms, namely reducing the interfacial tension (ift) between oil and water and altering the wettability of the rock toward a more water-wet condition. nevertheless, the performance of synthetic surfactants can still be improved, and the use of natural additives has become a widely developed approach because such materials are renewable, biodegradable, and non-toxic. pineapple peel (ananas comosus) is an agricultural waste product containing saponin, a surface-active compound that has the potential to act as a natural co-surfactant. the purpose of this research is to examine the effect of adding pineapple peel extract as a co-surfactant to an alpha olefin sulfonate (aos) surfactant solution in eor applications. the objectives of this research are to determine the effect of surfactant concentration variation on the performance of the solution with the addition of pineapple peel extract, to analyze the ability of the solution to reduce the ift value, to evaluate the effect of pineapple peel extract addition on rock wettability, and to determine the performance of the solution in increasing the recovery factor (rf). the research was conducted at laboratory scale using aos surfactant at concentrations of 0.3, 0.5, 0.8, 1.1, 1.4, and 1.7%, both without and with the addition of pineapple peel extract at a fixed concentration of 1%. the solutions were prepared using brine with a salinity of 12,500 ppm, while the hydrocarbon fluid used was light crude oil with an api gravity of 43°. the rock medium used was a sandstone core with a permeability of 170 md and a porosity of 19%. all tests were carried out at a temperature of 60°c. the testing stages included characterization of the pineapple peel extract through fourier transform infrared spectroscopy (ftir) analysis and phytochemical testing, followed by density measurement, aqueous stability, phase behavior, ift measurement using a spinning drop tensiometer, contact angle measurement using a contact angle meter, and core flooding tests with a scenario of 2 pv waterflooding, 1 pv surfactant injection, and 2 pv post flush. the ftir characterization results show that the pineapple peel extract exhibits absorption patterns similar to the saponin spectrum in three main regions, namely the o–h group at 3313.71–3275.13 cm⁻¹, the aliphatic c–h group at 2920.23 cm⁻¹, and the c–o–c group at 1099.43 and 991.41 cm⁻¹. the presence of these groups indicates a compound possessing both hydrophilic and hydrophobic parts. this indication was confirmed through phytochemical testing, in which the foam formed persisted up to the tenth minute in both the 10% stock extract and the 1% extract, confirming the presence of saponin in the pineapple peel extract. the density test shows that the density of the solution increases with increasing surfactant concentration in both systems, with the density of the aos solution ranging from 0.9924 to 0.9941 g/cc and that of the aos solution with pineapple peel extract ranging from 0.9918 to 0.9934 g/cc. in the aqueous stability test, all solutions remained clear without any color change or precipitation over three days at 60°c, indicating that the addition of 1% pineapple peel extract does not disturb the physical stability of the solution. the phase behavior test shows that in the aos system without additive, the concentration of 1.1% produced the highest total emulsion of 0.60%, whereas in the system with pineapple peel extract all concentrations produced a total emulsion of 0.40%, with the 1.4% concentration forming a middle-phase emulsion. based on these results, the aos concentration of 1.4% was selected for the subsequent testing stages. the ift measurement shows that the aos 1.4% solution produced a value of 1.91 dyne/cm, while the addition of 1% pineapple peel extract reduced the ift value to 1.85 dyne/cm, a reduction of 3.14%. despite this reduction, both values remain in the order of 10⁰ dyne/cm and have not reached the ultra-low interfacial tension condition. the wettability test shows that the aos 1.4% solution produced a θ water of 44.79°, while the addition of 1% pineapple peel extract produced a θ water of 54.22°. both values are below 90°, so the rock surface remains classified as water-wet. the core flooding results show that the waterflooding stage produced an rf of 56.25% for the aos 1.4% core and 49.29% for the aos 1.4% with 1% pineapple peel extract core. in the surfactant injection stage, the formulation with pineapple peel extract provided an additional recovery of 36.43% of ooip, higher than the aos without additive, which provided only 27.50%. the total recovery factor reached 85.71% for the formulation with pineapple peel extract and 83.75% for the formulation without additive, with final remaining oil of 14.29% and 16.25% of ooip, respectively. based on these results, it can be concluded that the addition of 1% pineapple peel extract as a co-surfactant is able to improve the performance of the aos 1.4% surfactant solution, as shown by the reduction in ift value, the formation of a middle-phase emulsion, and the increase in oil recovery across all parameters of the surfactant injection stage, while the water-wet character of the rock is maintained. therefore, pineapple peel extract has the potential to be utilized as a natural co-surfactant to support the performance of aos surfactant in eor methods, although the formulation still requires further optimization in order to achieve a greater reduction in ift.